The present invention relates to foamable modified polystyrene resin particles, a process for preparation of the resin particles, a foamed article which is produced by using the resin particles and has excellent balance of break resistance and cushioning property and a process for production of the foamed article.
It is well known that a foamed article produced by expanding foamable polystyrene resin particles obtained by impregnating polystyrene resin particles with a blowing agent is broken easily. To improve such a defect, there has been proposed a process for mechanically blending a styrene-butadiene block copolymer with a polystyrene resin (cf. JP-B-47-17465 and JP-A-54-158467), a process for mixing a styrene-butadiene block copolymer and a polybutadiene resin in a solvent to obtain a phase-separated microstructure by a solvent casting method (cf. JP-A-56-67344) and a process employing particles of a resin, so-called high impact polystyrene resin (HIPS) particles which are excellent in impact strength and are prepared by polymerizing a styrene monomer in the presence of conjugated diene polymer (JP-B-47-18428 and JP-A-7-90105).
However it is necessary that those foamable polystyrene resin particles are subjected to pelletizing through extrusion molding, which results in increase in cost. Further in case of a commercially available high impact polystyrene resin, a molecular weight of its polystyrene cannot be increased beyond a certain level.
As a method for improving break resistance at low cost, an impregnation polymerization method for polymerizing by impregnating styrene polymer particles with a conjugated diene monomer, or the like.
For example, JP-A-6-49263 discloses foamable polystyrene resin particles prepared by copolymerizing a monomer mixture of a styrene monomer and a conjugated diene monomer in a state of the monomer mixture being impregnated around the surface portion of polystyrene resin particles to give polystyrene resin particles containing the obtained conjugated diene copolymer gathering densely around the surface portion of the resin particle, and then impregnating the polystyrene resin particles with a blowing agent. Also it is disclosed that the center portion of the foamable resin particle is rich with polystyrene and, since the blowing agent is kept in the center portion, the resin particles having high expansion ratio can be obtained. Further it is disclosed that the monomer mixture is a mixture of 10 to 55% by weight of styrene monomer and 90 to 45% by weight of conjugated diene monomer. However in all the examples thereof, the conjugated diene monomer is used in an amount of not less than 1.5 times that of the styrene monomer and an object of that invention is to obtain polystyrene resin particles, in which a conjugated diene copolymer gathers densely around the surface portion of the resin particle and the center portion of the foamable resin particle is rich with polystyrene. From that point of view, only a monomer mixture comprising the conjugated diene monomer in a larger proportion essentially is disclosed.
The process mentioned above does not require a pelletizing step and is excellent from the viewpoint of cost. However the obtained foamed article, particularly the foamed article produced by molding pre-expanded particles within a mold is low in percentage of fused particles (fusion rate) and is insufficient in break resistance (in impact strength by a falling weight method) because the conjugated diene polymer particles are present densely around the surface portion of the resin particle.
Also in WO98/29485, there is disclosed foamable modified polystyrene resin particles obtained by polymerizing a conjugated diene monomer in polystyrene resin particles to form rubber particles of conjugated diene polymer in a uniformly dispersed state in the polystyrene resin particles, impregnating and polymerizing a styrene monomer in the obtained polystyrene resin containing rubber particles and then impregnating the obtained modified polystyrene resin with a blowing agent. Those foamable resin particles are characterized in that when they are expanded, there is substantially no deformation of the rubber particles before and after the expansion and an Izod impact strength of the modified polystyrene resin itself before expanded is lower than that of conventional resin called high impact polystyrene (HIPS). Also though the publication WO98/29485 discloses impregnation and polymerization of a styrene monomer and a conjugated diene monomer, a mixing ratio thereof disclosed is only 5:10 (weight ratio) in example of the publication. In this case, too, a monomer mixture being rich with the conjugated diene monomer is used.
The process mentioned above does not necessitate a pelletizing step and is excellent from the viewpoint of cost. Also a fusion rate of pre-expanded particles in a foamed article obtained by expanding them in mold is high and a break resistance (impact strength by a falling weight method) is improved as compared with the foamed article disclosed in the above-mentioned JP-A-6-49263 but an obtained foamed article is not equal to a foamed article of commercially available high impact polystyrene (HIPS) in case of expanding at high expansion ratio (about 50 times or more) into a foamed article having a density of not more than 0.02 g/cm3.
As mentioned above, the impregnation polymerization method does not necessitate a pelletizing step and can enhance break resistance of polystyrene resin foamed article at low cost. However it was difficult to attain break resistance equal to that of a foamed article obtained by using high impact polystyrene (HIPS) particles.
An object of the present invention is to provide foamable modified polystyrene resin particles which can be produced without a pelletizing step and gives a foamed article having excellent break resistance and cushioning property even at high expansion ratio, a process for preparation thereof and a foamed article obtained by expanding the resin particles.
Namely the present invention relates to foamable modified polystyrene resin particles which are foamable modified polystyrene resin particles (d) comprising modified polystyrene resin particles (c) containing a blowing agent (e); the modified polystyrene resin particles (c) comprise rubber polymer particles (b) dispersed in a polystyrene resin,
wherein the rubber polymer particles (b) are dispersed uniformly throughout a continuous phase of the polystyrene resin or in a state of a density of the particles (b) being higher in a center portion of the resin particle than in a surface layer portion thereof, and
the modified polystyrene resin particles (c) are modified polystyrene resin particles (c1) obtained by impregnating a monomer mixture consisting essentially of a styrene monomer and a conjugated diene monomer into polystyrene resin particles (a) and carrying out copolymerization, said monomer mixture being containing the styrene monomer in a larger amount than the conjugated diene monomer.
The modified polystyrene resin particles (c) may be modified polystyrene resin particles (c2) obtained by impregnating the monomer mixture consisting essentially of the styrene monomer and the conjugated diene monomer in the polystyrene resin particle (a) and carrying our copolymerization; said monomer mixture containing the styrene monomer in a larger amount than the conjugated diene monomer to give the modified polystyrene resin particles (c1) and further subjecting the obtained polystyrene resin particles to impregnation polymerization of a styrene monomer.
It is preferable that the above-mentioned rubber polymer particles (b) consists essentially of a copolymer of the styrene monomer and the conjugated diene monomer.
It is preferable that the modified polystyrene resin particles (c1) are prepared by impregnating 40 to 10 parts by weight of the monomer mixture into 60 to 90 parts by weight of polystyrene polymer particle (a) and carrying out copolymerization; said monomer mixture comprising the styrene monomer of more than 55% by weight and not more than 90% by weight and the conjugated diene monomer of more than 10% by weight and less than 45% by weight.
Provided that when the foamable modified polystyrene resin particles (d) are molded into a modified polystyrene resin foamed article having a density of 0.02 g/cm3, a cushioning coefficient of the foamed article is represented by A and a 50% failure height thereof according to falling weight method is represented by B, and that when foamable un-modified polystyrene resin particles (f) prepared by impregnating the polystyrene resin particles (a) with the blowing agent (e) are molded into an un-modified polystyrene resin foamed article having a density of 0.02 g/cm3, a cushioning coefficient of the foamed article is represented by C and a 50% failure height thereof according to falling weight method is represented by D, it is preferable that the modified polystyrene resin foamed article has a relation that A/C is 1.00 to 1.08 and B/D is 1.35 to 2.35.
Also it is preferable that the rubber polymer particles (b) being present in the center portion of the foamable modified polystyrene resin particles (d) have a circle equivalent diameter of average area of 0.01 to 0.20 xcexcm or a gel content of the modified polystyrene resin particles (c) is 15 to 40% by weight.
Further it is preferable that the above-mentioned rubber polymer particles (b) are flattened in cell membrane of the modified polystyrene resin foamed article obtained by expanding the foamable modified polystyrene resin particles, and particularly an average flatness of the rubber polymer particles (b) flattened in a cell membrane of the modified polystyrene resin foamed article obtained by expanding the foamable modified polystyrene resin particles (d) so that the foamed article has a density of 0.02 g/cm3 is within a range of 1.1 to 9.
Further the present invention relates to foamable modified polystyrene resin particles which are foamable modified polystyrene resin particles (d) comprising modified polystyrene resin particles (c) containing a blowing agent (e); the modified polystyrene resin particles (c) comprise rubber polymer particles (b) dispersed in a polystyrene resin,
wherein the rubber polymer particles (b) are dispersed uniformly throughout a continuous phase of the polystyrene resin or in a state of a density of the particles (b) being higher in a center portion of the resin particle than in a surface layer portion thereof, and
provided that when the foamable modified polystyrene resin particles (d) are molded into a modified polystyrene resin foamed article having a density of 0.02 g/cm3, a cushioning coefficient of the foamed article is represented by A and a 50% failure height thereof according to falling weight method is represented by B, and that when foamable unmodified polystyrene resin particles (f) prepared by impregnating polystyrene resin particles (a) with a blowing agent (e) are molded into an un-modified polystyrene resin foamed article having a density of 0.02 g/cm3, a cushioning coefficient of the foamed article is represented by C and a 50% failure height thereof according to falling weight method is represented by D, the modified polystyrene resin foamed article has a relation that A/C is 1.00 to 1.08 and B/D is 1.35 to 2.35.
Also in those foamable modified polystyrene resin particles, it is desirable that the above-mentioned rubber polymer particles (b) are particles of a copolymer consisting essentially of the styrene monomer and the conjugated diene monomer.
Also it is preferable that the rubber polymer particles (b) being present in the center portion of the foamable modified polystyrene resin particles (d) have a circle equivalent diameter of average area of 0.01 to 0.20 xcexcm or a gel content of the modified polystyrene resin particles (c) is 15 to 40% by weight.
Still further the present invention relates to foamable modified polystyrene resin particles which are foamable modified polystyrene resin particles (d) comprising modified polystyrene resin particles (c) containing a blowing agent (e); the modified polystyrene resin particles (c) comprise rubber polymer particles (b) dispersed in a polystyrene resin,
wherein the rubber polymer particles (b) are dispersed uniformly throughout a continuous phase of the polystyrene resin or in a state of a density of the particles (b) being higher in a center portion of the resin particle than in a surface layer portion thereof, and
the rubber polymer particles (b) are flattened in a cell membrane of a modified polystyrene resin foamed article obtained by expanding the foamable modified polystyrene resin particles (d).
It is desirable that the above-mentioned rubber polymer particles (b) are particles of a copolymer consisting essentially of the styrene monomer and the conjugated diene monomer.
Provided that when the foamable modified polystyrene resin particles (d) are molded into a modified polystyrene resin foamed article having a density of 0.02 g/cm3, a cushioning coefficient of the foamed article is represented by A and a 50% failure height thereof according to falling weight method is represented by B, and that when foamable un-modified polystyrene, resin particles (f) prepared by impregnating polystyrene resin particles (a) with a blowing agent (e) are molded into an un-modified polystyrene resin foamed article having a density of 0.02 g/cm3, a cushioning coefficient of the foamed article is represented by C and a 50% failure height thereof according to falling weight method is represented by D, it is preferable that the modified polystyrene resin foamed article has a relation that A/C is 1.00 to 1.08 and B/D is 1.35 to 2.35.
Also it is preferable that the rubber polymer particles (b) being present in the center portion of the foamable modified polystyrene resin particles (d) have a circle equivalent diameter of average area of 0.01 to 0.20 xcexcm or a gel content of the modified polystyrene resin particles (c) is 15 to 40% by weight.
It is preferable that an average flatness of the rubber polymer particles (b) flattened in a cell membrane of the modified polystyrene resin foamed article obtained by expanding the foamable modified polystyrene resin particles (d) and having a density of 0.02 g/cm3 is in a range of 1.1 to 9.
Also the present invention relates to the modified polystyrene resin foamed article having a cell membrane of modified polystyrene resin comprising a polyester resin and rubber polymer particles (b) dispersed in the resin,
wherein the rubber polymer particles (b) are flattened in the cell membrane, and an average flatness of the flattened rubber polymer particles (b) in the cell membrane of the modified polystyrene resin foamed article having a density of 0.02 g/cm3 is in a range of 1.5 to 8.
It is desirable that the above-mentioned rubber polymer particles (b) are particles of a copolymer substantially comprising the styrene monomer and the conjugated diene monomer.
Provided that a cushioning coefficient of the modified polystyrene resin foamed article having a density of 0.02 g/cm3 is represented by A and a 50% failure height thereof according to falling weight method is represented by B, and that a cushioning coefficient of a foamed article of unmodified polystyrene resin containing no rubber polymer particles and having a density of 0.02 g/cm3 is represented by C and a 50% failure height thereof according to falling weight method is represented by D, it is preferable that the modified polystyrene resin foamed article has a relation that A/C is 1.00 to 1.08 and B/D is 1.35 to 2.35.
Further the present invention relates to the modified polystyrene resin foamed article obtained by expanding the above-mentioned foamable modified polystyrene resin particles and the process for producing the modified polystyrene resin foamed article by expanding the above-mentioned foamable modified polystyrene resin particles.
Still further the present invention relates to the process for producing the foamable modified polystyrene resin particles which comprises:
impregnating a monomer mixture consisting essentially of the styrene monomer and the conjugated diene monomer and comprising the styrene monomer in a larger amount than the conjugated diene monomer, into the polystyrene resin particle (a) and carrying out copolymerization to give modified polystyrene resin particles (c1),
wherein rubber polymer particles (b) are dispersed uniformly throughout a continuous phase of the polystyrene resin or in a state of a density of the particles (b) being higher in a center portion of the resin particle than in a surface layer portion thereof, and then
impregnating the modified polystyrene resin particles (c1) with a blowing agent (e),
and further relates to the process for producing the foamable modified polystyrene resin particles which comprises:
impregnation-polymerizing the modified polystyrene resin particles (c1) with a styrene monomer to give the modified polystyrene resin particles (c2) and then
impregnating the modified polystyrene resin particles (c2) with the blowing agent (e).
In the impregnation polymerization of those production processes for obtaining the modified polystyrene resin particles (c1), it is preferable that 40 to 10 parts by weight of the monomer mixture is impregnated into 60 to 90 parts by weight of the polystyrene polymer particles (a) and the monomer mixture comprising a styrene monomer of more than 55% by weight and not more than 90% by weight and a conjugated diene monomer of more than 10% by weight and less than 45% by weight is used.
The xe2x80x9cfoamed articlexe2x80x9d used herein encompasses pre-expanded particles obtained by expanding the foamable resin particles and a molded article obtained by molding the pre-expanded particles in a mold, both of which are described hereinafter. Unless otherwise noted, the word xe2x80x9cfoamed articlexe2x80x9d means the both of them.
Prior to explaining the present invention, explanations of the words used in the present invention are made below.
In the present invention, the state of the rubber polymer particles (b) xe2x80x9cbeing dispersed uniformly throughout the polystyrene resin particle (a) or a density thereof being higher in a center portion of the resin particle than in a surface layer portion of the resin particlexe2x80x9d means that the distributing state of the rubber polymer particles (b) does not differ between the surface layer portion and the center portion of the polystyrene resin particle (a) or is sparser in the surface layer portion.
In the present invention, the xe2x80x9csurface layer portionxe2x80x9d means a portion defined between the depth of 2 xcexcm to 6 xcexcm from the surface of the modified polystyrene resin particle (c) toward the center thereof. The xe2x80x9ccenter portionxe2x80x9d means a sphere portion having a radius of 50 xcexcm, the center of said sphere portion being the center of the modified polystyrene resin particle, provided that the particle is a sphere.
The concrete measuring method is explained below by using photographs of the modified polystyrene resin particle prepared in Example 2 mentioned hereinafter, which were taken with a transmission electron microscope (TEM). FIG. 1 is a TEM photograph (xc3x9740000) of the xe2x80x9ccenter portionxe2x80x9d, and FIG. 2 is a TEM photograph (xc3x9740000) of the xe2x80x9csurface layer portionxe2x80x9d. FIGS. 3 (In FIG. 3, X mark represents the center of the particle) and 4 are photocopies of FIGS. 1 and 2, respectively for measuring an area percentage (%) of the rubber particles.
The area percentage (%) of the rubber particles is measured and calculated as follows.
First an area A (1.25 xcexcmxc3x971.25 xcexcm) surrounded by a full line in FIGS. 3 and 4 is enlarged by four times with a copying machine, respectively, and a weight thereof is measured. Black region (rubber polymer particles dyed with osmium oxide) are cut off (in case of rubber polymer particles containing occlusion polystyrene particles therein, a region of the rubber polymer particles containing the occlusion polystyrene particles is cut off as it is), and a weight of the cut portions B (or remaining portions after cutting off) is measured. The area percentage (%) of the rubber particles is represented by (cut portions B/total area A)xc3x97100. In FIG. 1 (FIG. 3) and FIG. 2 (FIG. 4), the area percentage is 38.4% and 25.5%, respectively.
Then the xe2x80x9carea ratio of rubber particlesxe2x80x9d of the xe2x80x9csurface layer portionxe2x80x9d to the xe2x80x9ccenter portionxe2x80x9d is calculated.                               Area          ⁢                      xe2x80x83                    ⁢          ratio          ⁢                      xe2x80x83                    ⁢          of                                              rubber          ⁢                      xe2x80x83                    ⁢          particles                      =                                          Area            ⁢                          xe2x80x83                        ⁢            percentage            ⁢                          xe2x80x83                        ⁢                          (              %              )                        ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            rubber                                                            particles            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            surface            ⁢                          xe2x80x83                        ⁢            layer            ⁢                          xe2x80x83                        ⁢            portion                                                                    Area            ⁢                          xe2x80x83                        ⁢            percentage            ⁢                          xe2x80x83                        ⁢                          (              %              )                        ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            rubber                                                            particles            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            center            ⁢                          xe2x80x83                        ⁢            portion                              
In the present invention, the state of xe2x80x9cbeing uniformly dispersed throughout or a density being higher in the center portion than in the surface layer portionxe2x80x9d is as mentioned above. From an aspect of xe2x80x9can area ratio of the rubber particlesxe2x80x9d, the area ratio is preferably from 0.10 to 1.15, more preferably 0.30 to 1.15, particularly preferably 0.50 to 1.15.
When the area ratio of rubber particles exceeds 1.15, there is a tendency that fusion between the pre-expanded particles at the time of in-mold molding is inferior. Since the foamable modified polystyrene resin particles of the present invention are uniform throughout or a density of the rubber polymer particles is higher in the center portion than in the surface layer portion, the fusion between the pre-expanded particles is easy and in-mold molding is easy.
In the present invention, the xe2x80x9ccushioning coefficient ratio (A/C)xe2x80x9d means the following value.
First the cushioning coefficient is a minimum cushioning coefficient when a 14 kg of a weight is dropped from a height of 60 cm onto a 90 mmxc3x9790 mmxc3x9750 mm sample of expanded foamed article and the sample is deformed by 50 to 65% as compared with the sample before the test according to JIS Z 0235. In the concrete measurement and calculation, first a reference cushioning coefficient C of a foamed article obtained by in-mold molding of un-modified polystyrene resin particles under the expansion-molding conditions mentioned hereinafter is measured, and then a cushioning coefficient A of a foamed article obtained by expansion-molding of polystyrene resin particles to be tested under the same expansion-molding conditions is measured. The A/C is called a cushioning coefficient ratio. The smaller the ratio is, the better the cushioning characteristic is.
The xe2x80x9cimpact strength ratio (B/D) according to the falling weight methodxe2x80x9d of the present invention means a value obtained as follows.
According to JIS K 7211, a 321 g steel ball is dropped on a 200 mmxc3x9720 mmxc3x9740 mm test piece of a foamed article and a 50% failure height which causes rupture of 50% of the number of test pieces of the foamed article is calculated by the following equation:       H    50    =            H      i        +          d      ⁡              [                                            ∑                              (                                  i                  ·                                      n                    i                                                  )                                      N                    ±          0.5                ]            
where H50: A 50% failure height (cm) which causes rupture of 50% of the number of test pieces of the foamed article
Hi: Test height (cm) when the height level (i) is zero, representing the height anticipated to cause the rupture of the test piece
d: Height interval (cm) for increasing or decreasing the test height
i: Height level which increases or decreases by one, taking the value of zero when the test height is H1 (i= - - - xe2x88x923, xe2x88x922, xe2x88x921, 0, 1, 2, 3, - - -)
ni: Number of test pieces which break (or do not break) at each height level
N: Total number of test pieces which break (or do not break) (N=xcexa3ni)
Whichever is the larger of the number of broken and unbroken test pieces shall be used. In case where the numbers are the same, either one may be used.
xc2x10.5: This value shall be negative when using the data when broken and be positive when using the data when unbroken.
In the concrete measurement and calculation, first a reference 50% failure height D of a foamed article obtained by in-mold molding of un-modified polystyrene resin particles under the expansion-molding conditions mentioned hereinafter is measured, and then a 50% failure height B of a foamed article obtained by expansion-molding of polystyrene resin particles to be tested under the same expansion-molding conditions is measured. The B/D is called an impact strength ratio according to the falling weight method. The larger the ratio is, the higher the break resistance is.
In the present invention, the xe2x80x9cdensityxe2x80x9d means the following value.
A density D (g/cm3) of the foamed article is obtained by the following equation according to JIS K 6767.
D=G/V 
wherein
G: Weight of foamed article (g),
V: Volume of foamed article (cm3)
V is calculated by cutting a part of the foamed article into a rectangular parallelepiped and measuring a depth, width and height thereof. Measuring tools and accuracy of measurement are according to JIS K 6767.
The expansion-molding conditions for the measurements of the above-mentioned impact strength ratio and cushioning coefficient ratio are as follows. Those conditions are used also in examples and comparative examples described hereinafter.
Pre-Expansion Conditions
Foamable resin particles
Average particle size: 1 mm
Blowing agent: Pentane (normal/iso=40/60)
A normal pressure pre-expanding equipment which was pre-heated is charged with 1 to 2 kg of foamable resin particles, followed by passing steam at about 0.061 MPa and introducing air properly with stirring. Thus the particles are expanded up to a given expansion ratio in about 1 minute to about 3 minutes.
In-Mold Molding Conditions
After molding under the following conditions, a foamed article is allowed to stand for vacuum cooling.
Molding machine: TH90VMII available from Toyo Kikai Kinzoku Kabushiki Kaisha
Density after molding: 0.02 g/cm3 (expansion ratio: about 50 times)
Molding conditions:
Pre-steaming: 3 seconds
One-way steaming: 4 seconds
Opposite steaming: 1 second
Autoclave steaming: 12 seconds
Additional steaming: 3 seconds
Retaining: 3 seconds
Water cooling: 20 seconds
Set steam pressure at the heating from both sides:
Cavity/core=0.066/0.087 (MPa)
In the present invention the xe2x80x9caverage flatnessxe2x80x9d of the rubber polymer particles (b) in the cell membrane of the foamed article is determined as follows. Those conditions are used also in examples and comparative examples described hereinafter.
The foamable resin particles are expansion-molded by the above-mentioned method to give a foamed article having a density of 0.02 g/cm3. The rubber polymer particles in the cell membrane of the foamed article are dyed with osmium oxide, followed by viewing with a transmission electron microscope (JEM-1200EX available from Nippon Denshi Kabushiki Kaisha, xc3x977200 to xc3x9740000). Then with respect to 100 flattened rubber polymer particles, a dimension thereof in the cell membrane direction and a dimension in a thickness direction thereof are measured. A value obtained by dividing the dimension in the cell membrane direction by the dimension in the thickness direction is assumed to be a flatness. An average value of the flatness of the rubber polymer particles having a flatness of not less than 1.1 is assumed to be an average flatness.
In the present invention the xe2x80x9ccircle equivalent diameter of average areaxe2x80x9d of the rubber polymer particles (b) in the center portion of the foamable modified polystyrene resin particle (d) is determined as follows. Those conditions are used also in examples and comparative examples described hereinafter.
The rubber polymer particles in the foamable resin particle are dyed with osmium oxide, and with a transmission electron microscope (JEM-1200EX available from Nippon Denshi Kabushiki Kaisha, xc3x9740000), a photograph was taken. An area of 3.76 xcexcm2 calculated from a scale (0.5 xcexcm) on the photograph was enlarged by four times and photocopied and a black rubber portion dyed with osmium oxide of the photocopy is cut off (in case of rubber polymer particles containing occlusion polystyrene particles therein, a region of the rubber polymer particles containing the occlusion polystyrene particles is cut off as it is), and a weight E g of all the cut portions of rubber polymer particles is measured. Then the weight E g of the cut pieces of paper is divided by the number F of rubber polymer particles and an average weight G g of the rubber polymer particles is calculated. Separately a portion of the photocopy including the scale (0.5 xcexcm) was enlarged by four times similarly and photocopied. The enlarged portion equivalent to 0.25 xcexcm2 is cut off and its weight is measured to determine a weight H g of the paper equivalent to 0.25 xcexcm2. An area per 1 g of paper, i.e. I xcexcm2/g is calculated by dividing 0.25 xcexcm2 by the paper weight H g equivalent to 0.25 xcexcm2. Then an average area J xcexcm2 of the rubber polymer particles is calculated by multiplying the average weight G g of the rubber polymer particles by the area per 1 g of paper, i.e. I xcexcm2/g. This average area J xcexcm2 of the rubber polymer particles is divided by number xcfx80 and a root of the obtained value is multiplied by 2. The thus obtained value is assumed to be a circle equivalent diameter of average area K xcexcm. Herein the measurements were made by using FIGS. 1 and 2 used for obtaining the rubber portion area ratio.
The xe2x80x9ccenter portionxe2x80x9d means a sphere portion having a radius of 50 xcexcm, the center of said sphere portion being the center of the foamable modified polystyrene resin particle, provided that the particle is a sphere.
In the present invention the xe2x80x9cgel contentxe2x80x9d is obtained as follows.
0.5 Gram of foamable modified polystyrene resin particles is put in a solution comprising 31.8 ml of methyl ethyl ketone and 3.2 ml of methanol. After stirring for eight hours or more, a portion of solution (dissolved portion) is separated from a gel portion (insoluble portion) by centrifugal separation (for 30 minutes at 15,000 rpm), and the portion of solution is removed. Then a solution comprising 31.8 ml of methyl ethyl ketone and 3.2 ml of methanol is added, followed by stirring at 30xc2x0 C. for 30 minutes and separating a portion of solution from a gel portion by centrifugal separation (for 30 minutes at 15,000 rpm). The portion of solution is removed. Further a solution comprising 31.8 ml of methyl ethyl ketone and 3.2 ml of methanol is added, followed by stirring at 50xc2x0 C. for 30 minutes and separating a portion of solution from a gel portion by centrifugal separation (for 30 minutes at 15,000 rpm). Then the portion of solution is removed and the obtained gel portion is dried at 60xc2x0 C. for eight hours to measure a weight of the gel portion. Thus a weight of the gel portion contained in 0.5 g of foamable modified polystyrene resin particles is calculated. The obtained value is assumed to be a gel content.